Interferometry is a useful technique for measuring one or more spatial characteristics of a sample, such as a semiconductor wafer or any other semiconductor or non-semiconductor substrate, based on information associated with illumination reflected from test surfaces of the sample. As semiconductor fabrication continually requires higher levels of accuracy and precision, improved interferometry techniques are needed to meet the demands of modern fabrication technologies. Phase-shifting interferometry offers many advantages over other interferometry techniques including, but not limited to, higher measurement accuracy, insensitivity to contrast, illumination uniformity, and obtainable phase at fixed grid points. It would be desirable to provide an improved method and system for performing interferometry on a sample.